Carotenoid bioaccessibility in fruit and vegetable based systems

Carotenoids are important micronutrients that have been linked with several health benefits. To exert the health benefits, carotenoids must be absorbed and reach their site of action. The absorption process of carotenoids consists of various steps including release from the food matrix, solubilization into a lipid phase, incorporation into micelles (i.e. bioaccessibility) and uptake through the intestine epithelia. However, previous investigations have proven that the transfer of carotenoids into the oil phase during digestion that is a prerequisite for their bioaccessibility is limited by matrix related factors that hamper carotenoid release and their transfer into oil. Consequently, the fraction of carotenoids that is actually absorbed, and serves health related functions (i.e. bioavailability), was also proven to be generally low. In this context, high pressure homogenization (HPH) and thermal processing in the presence of oil were investigated as alternative strategies that might improve carotenoid transfer from fruit and vegetable matrices to oil phase and potentially enhance their bioaccessibility.

Hereto, the effect of HPH (10, 30, 50, 70, 100 MPa) and thermal processing (80-120 °C, 20 min; 95-110 °C, 0-40 min) on the transfer of carotenoids (lycopene, α-carotene and β-carotene) from tomato and carrot based matrices to the oil phase was investigated. The effect of carotenoid bioencapsulation in determining carotenoid release from the matrix and subsequent transfer into the oil phase was investigated by decomposing tomato and carrot matrices into a chromoplast and a cell cluster fraction. Subsequently, the effect of storage conditions on the stability of carotenoids and lipids in shelf-stable tomato and carrot-based systems was investigated. Finally, the kinetics of the micellar incorporation of lycopene and its cis isomers, β-carotene and its cis isomers as well as α-carotene and lipid digestion products, free fatty acids (FFAs) and monoglycerides (MAGs) during in vitro digestion of low-fat oil in water emulsions was investigated. Moreover, the relation between carotenoid bioaccessibility and lipid digestion was elucidated.

During HPH, carotenoid transfer to oil was clearly increased by increasing homogenization intensity. This is the result of the increased shearing and contact between the matrix and the oil phase, which could facilitate the transfer of carotenoids from the matrix into oil. During thermal processing treatment conditions of >100 °C for 10 min were necessary to significantly favour carotenoid transfer to oil (≥75%). HPH at ≥ 50 MPa could achieve a similar carotenoid transfer efficiency in the chromoplast fractions. The cell wall represented a limiting factor for carotenoid transfer during processing. This suggests that in order to achieve maximum carotenoid transfer to oil, the food systems should be disrupted to a level where the cell wall is broken. The selective transfer of a particular carotenoid during processing depended on its chemical structure (e.g. transfer of lycopene<β-carotene).

During storage (6 months in the dark at 20, 30 and 40 °C) of carrot and tomato matrices processed (HPH at 100 MPa, F0 = 5 min) in the presence of 5% olive oil, carotenoids exhibited high stability with retention of ≥ 97%. Furthermore, these processing conditions could improve the all-trans lycopene as well as all-trans α- and β-carotene in vitro bioaccessibility, which remained unchanged during storage. This suggests that these food systems, rich in antioxidants, and the combination of processing and storage conditions, including limited headspace and absence of light, potentially promoted the protection of both the lipid fraction and carotenoids therein.

Further experimental work revealed for the first time, the kinetics of the micellar incorporation of lycopene and its cis isomers, β-carotene and its cis isomers and α-carotene as well as FFAs and MAGs during in vitro digestion of low-fat oil-in-water emulsions. The release in the digest and incorporation into micelles of FFAs and MAGs were not influenced by the type and amount of carotenoids initially present in the emulsions. However, carotenoid micellar incorporation depended on the micellar incorporation of lipid digestion products (FFA+MAGs) and this dependency varied as a function of carotenoid type. Concerning the geometry of the carotenoid molecule, it was hypothesized that isomeric configurations having a bent structure, with the bend close to the center of the polyene chain, can be better accommodated within the micelles compared to their all-trans counterparts. Independently of the matrix in which all-trans β-carotene was present or its concentration in the initial emulsion, similar rate constants were found for its micellar incorporation suggesting that these kinetic parameters can be strongly linked to the structural characteristics of the carotenoid.

In conclusion, the present work indicates that thermal processing and HPH of carotenoid-rich fruits and vegetables in the presence of oil can facilitate the transfer of carotenoids to the oil phase and potentially their bioaccessibility. This can be crucial to improve the nutritional quality of carrot and tomato-based products. A first order fractional conversion kinetic model enabled reliable prediction of micellar incorporation of different carotenoids and lipid digestion products during in vitro digestion of low-fat emulsions. Consequently, the kinetic approach applied in this work was proven to be a useful tool to study the effect of process and product-related factors on micronutrient (i.e. carotenoids) bioaccessibility.